Bluetooth: PACS: Improve handling of notify on reconnect

When we reconnect to a bonded device we always attempted to
send the changed PACS notifications, regardless of
whether the remote device has subscribed to them or not.

This resulted in a lot of failed attempts to send
notifications when
CONFIG_BT_GATT_ENFORCE_SUBSCRIPTION is enabled (which it is
by default).

Ideally we would not set the flags for devices that have not
subscribed, but since we cannot call bt_gatt_is_subscribed for
non-connected devices, we need to do this when the notify_cb
is called.

It would be possible to use BT_GATT_CCC_MANAGED instead
of BT_AUDIO_CCC to keep track of the writes for each client,
but that would be a minor optimization which would require
significant rework and additions to PACS. The current
solution is inspired by the implementation used in has.c

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>